Maybe this has been done before...
Sorry if it has, but on to the point:
Half-Life and Half-Life 2 are interwound with very interesting storylines if you care to look for it. Without a doubt everybody knows about te G-Man, but nobody knows who he is or why he's doing what he's doing. Nor do we know much else, since we play a manipulated pawn in both games and all of the expansions.
Questions:
A) Does Half-Life 2 take place on Earth?
B) If so, how far in the future is it?
I ask this because Half-Life was supposed to occur down here in New Mexico, around the time the game was actually released. Half-Life 2 seems like an alien world, not just futuristic. The citizens are completed used to Headcrabs, Zombies, and Antlions. All the technology in Half-Life 2 would take quite a while to develop and mass produceto the extent done in Half-Life 2.
C) What happen to Xen in Half-Life 2?
In Half-Life, you get to travel to and fro Xen, and meet all its inhabitants whether they be invading or vice versa. In Half-Life 2, the only things from Xen we encounter are Headcrabs and possibly Striders and Antlions. We don't even get to go to Xen in 2.
D) Where is the Combine getting their resources?
When all of the population revolts in City 17, it is highly probably that all te other cities would revolt. Therefore, the government of the cities has just lost its entire workforce. How do they supply the Combine Soldiers? I'm thinking his extraterrestial contact near the end of the game, but I could be wrong.
E) Who is Breen talking with?
In Half-Life, Gordon killed Nihilanth and freed the slaves (Vortigaunts). Therefore, Xen ought to be in total anarchy...

I'd write a lot more, but I got to get to school. Be back this afternoon.